1. Field of the Invention
This invention relates to a pneumatic radial tire controlling a complicated motion of the tire exceeding a driver's forecast or an occurrence of so-called wandering phenomenon to largely improve straight running stability when a vehicle is run on a slant face of an uneven road surface such as rut or the like.
2. Description of Related Art
The radial tire is excellent in the wear resistance and the steering stability because carcass cords are arranged to extend substantially in a direction perpendicular to an equatorial plane of the tire. Therefore, radial tires are frequently used as compared with bias tires in not only passenger cars but also vehicles such as small-size truck, truck and bus and the like with a recent advance of high-speed performance of the vehicle.
However, high-speed running of the vehicles is daily carried out in accordance with the improvement and expansion of road systems and the like. For this end, it is strongly demanded to sufficiently control the wandering phenomenon liable to be caused in the radial tire rather than the bias tire to thereby enhance the straight running stability of the radial tire and to further improve safety.
Viewing the occurrence of wandering phenomenon in the radial tire, as shown in FIG. 1, when the tire T is rotated on a slant face S of rut or the like, each of load W, reaction force F.sub.R from road surface and camber thrust F.sub.C is applied to the tire T and hence lateral force F.sub.Y as a sum of horizontal components of these forces acts thereto. The tire of radial structure is high in the treading rigidity as compared with the tire of bias structure, and also the rigidity of the tread portion is fairly higher than that of the side portion owing to the structure of the radial tire, so that the camber thrust F.sub.C is made smaller than that of the bias tire and hence the lateral force F.sub.Y directing to a lower side of the slant face S becomes relatively large by a quantity corresponding to the reduced quantity of the camber thrust F.sub.C. As a result, the tire T strongly tends to slip downward on the slant face and it is difficult to ride over the rut and hence the wandering phenomenon is caused.
As shown at radial section of the tire in FIG. 2, when the tire is rotated on the slant face S, the camber thrust F.sub.C is caused based on the fact that the tread portion T.sub.r tends to strongly contact with the ground at an upper side or a mountain side of the slant face S and rise upward at a valley side of the slant face S. Particularly falling-down deformation of a region B.sub.u in the vicinity of the buttress of the tire side portion or so-called bulging deformation bside toward the mountain side is caused at the mountain side due to the strong ground contact of the tread portion T.sub.r. This bulging deformation bside brings about bending deformation of the tread portion T.sub.r near a ground contact end or so-called pushing-out deformation bsho, and such a pushing-out deformation bsho in a zone near to the ground contact end further gives shearing deformation S.sub.S as shown by dotted lines in FIG. 2 to the tread rubber in a portion near to the ground contact end. This shearing deformation S.sub.S creates a lateral force F.sub.CS directing to the upper side of the slant face S.
As regards the camber thrust F.sub.C, in the bias tire having a small difference of rigidity among constitutional parts of the tire and a relatively soft structure, sufficiently large bulging deformation bside and pushing-out deformation bsho can be insured and hence the camber thrust F.sub.C becomes large. Since the rigidity of the tread portion in the radial tire becomes considerably higher than the rigidity of the side portion, a tendency of creating escape deformation in a direction opposite to the above as exaggeratedly shown by a phantom line in FIG. 2 becomes strong in the tire side portion and hence the shearing deformation S.sub.S of the tread rubber becomes naturally small and it is obliged to reduce the camber thrust F.sub.C.
In order to increase the camber thrust F.sub.C in the pneumatic radial tire, therefore, it is effective that the bulging deformation bside of the region B.sub.U is made large and the transmission efficiency of the bulging deformation bside is enhanced to make the pushing-out deformation bsho in the zone near to the ground contact end to thereby increase the shearing deformation S.sub.S of the tread rubber at the ground contact end, or it is effective that the ground contact area of the tire to the slant face S is made large to increase a total quantity of the lateral force F.sub.CS directed to the upper side of the slant face S.